Hybrid Membrane/Distillation Method and System for Removing Nitrogen from Methane

ABSTRACT

A hybrid gas separation membrane/cryogenic distillation method and system produces high purity gaseous methane from a gas mixture containing a majority of methane and a minority of nitrogen.

CROSS-REFERENCE TO RELATED APPLICATIONS

None.

BACKGROUND

Prior art in nitrogen removal from natural gas includes several references to cryogenic separation. With adequate feed pressure, the single column process can perform the separation using no external energy other than power for a liquid pump which is used to pump liquid methane to the desired product pressure. Single and dual pressure columns are common practice in cryogenic applications such as nitrogen rejection from a natural gas stream.

U.S. Pat. No. 4,878,932 describes a single column process wherein the cooled feed is pre-separated in a phase separator into vapor and liquid portions, the vapor is condensed and at least partly employed as reflux for the column. This single column process scheme tends to have good recovery when the N₂ content in the feed stream is high, typically more than 20%. However, when the N₂ content decreases, the methane recovery tends to fall sharply.

For natural gas streams having a relatively low N₂ content, dual distillation columns operated at different pressures typically are used to maintain high recovery. U.S. Pat. No. 4,415,345 describes one example of a double column system. Generally speaking, in dual distillation columns the high pressure column provides a methane enriched stream which is sent to the low pressure column for further enrichment. Liquid methane product is then pumped to the desired product pressure. More particularly, the double column system is operated such that condenser duty to the first column provides the reboiler duty of the second column whereas in the single column process, heat integration is carried out by using reboiler duty to condense the feed to the distillation column. In either of the single or double column schemes, the only significant external energy that is required is in the form of a liquid pump. For feed gas pressures of 80 bar or higher and methane product pressures of up to 35 bar, no additional cooling or compression is typically required. The Joule-Thomson effect between the feed gas and product streams is sufficient to satisfy plant refrigeration requirement.

A typical example of a dual column system is shown in FIG. 1. According to this scheme, a high pressure N₂-containing natural gas feed 1 (typically at a pressure of about 80 bar) is cooled at heat exchanger 5 by heat-exchange with methane stream 13, high pressure N₂ stream 17 and low pressure N₂ stream 21 The cooled feed 9 is then expanded at Joule-Thomson valve 25 yielding lowered pressure feed 29 which is sent to the high pressure distillation column 33. Distillation column 33 fractionates feed 29 into a methane-rich liquid component carried in stream 41 and a high pressure N₂-rich vapor component 37. Condenser-reboiler 82 condenses a portion 38 of the vapor component 37 to provide a liquid stream rich in N₂ 53. A portion 17 of 37 can be recovered as high pressure gaseous N2 stream 17. A portion 55 of 53 is sent back to column 33 as reflux. The remaining portion 57 is then directed to column 81. Streams 41, 57 are cooled at heat exchanger 61 through heat exchange with liquid methane stream 65 and low pressure gaseous N₂ stream 69 before being directed to the low pressure distillation column 81. The low pressure column 81 fractionates the methane/N₂ mixture contained therein into low pressure gaseous N2 stream 69, and high purity liquid methane stream 78. Stream 76 is directed to the condenser-reboiler 82 which receives heat from stream 37 and returns a stream of vaporized or partially vaporized methane 84 to column 81. A liquid pump 93 receiving high purity liquid methane 90 from the bottom of column 81 pumps high purity liquid methane stream 65 through heat exchanger 61 whereat it and the high pressure gaseous N₂ stream 69 are warmed. Liquid methane stream 13 is then vaporized in exchanger 5 to provide a stream of high purity methane 95. Low pressure N₂ stream 21 and high pressure N₂ stream 17 are warmed at heat exchanger 5 to provide streams of low pressure N₂ 99 and high pressure N₂ 97, respectively.

FIG. 2 shows a typical example of a single column separation scheme. Here the feed 101 is cooled in the heat exchanger 105 through heat exchange with streams 112, 116. The cooled stream 109 is expanded in an expansion valve (or also called Joule-Thomson valve) 120 to lower pressure. This reduction of pressure results in a two-phase stream which is then phase separated into vapor and liquid streams at phase separator 124. The vapor stream 128 is condensed at condenser-reboiler 182. The consensed vapor stream 149 is cooled at heat exchanger 161 through heat exchange with liquid methane stream 165 and high pressure gaseous N₂ stream 169 and sent to the distillation column 181 as reflux. The liquid stream 142 from the separator 124 is subcooled at heat exchanger 161 (also through heat exchange with liquid methane stream 165 and high pressure gaseous N₂ stream 169) and directed to the column 181. A stream of high purity liquid methane 176 is directed to the condenser-reboiler 182 which receives heat from stream 128 and returns a stream of vaporized or partially vaporized methane 184 to column 181. A liquid pump 193 receiving high purity liquid methane 190 from the bottom of column 181 pumps high purity liquid methane stream 165 through heat exchanger 161 whereat it and the high pressure gaseous N₂ stream 169 are warmed. Liquid methane stream 112 is then vaporized in exchanger 105 to provide a stream of high purity methane 194. High pressure N₂ stream 116 is warmed at heat exchanger 105 to provide stream of high pressure N₂ 196.

Membranes have been used in hybrid application such that the feed is first sent to the membrane, the product of which is then sent to a distillation column for separation. There is also prior art available on use of membrane-distillation hybrid system for natural gas applications, such as U.S. Pat. No. 5,647,227.

While the above approaches provide sufficient solutions for purifying many types of N₂-containing natural gas, they often suffer from one or more disadvantages. For cryogenic separation units, variation in the feed N₂ content can pose problem to the operation of a cryogenic separation unit. This is because while single column distillation systems work well for high N₂ content natural gas, recoveries can fall sharply as the N₂ content is decreased. In such cases, a second column may be necessary. This adds to the capital cost.

Thus, it is the object of the current invention to provide a scheme which can provide sufficient methane recovery for feeds having variable N₂ contents and requires minimal energy input.

SUMMARY

There is provided a method of purifying a gas mixture having a majority of methane and a minority of nitrogen. It includes the following steps. The gas mixture is cooled. The cooled gas mixture is fed to a gas separation membrane to provide a permeate stream further enriched in methane and a residue stream further enriched in nitrogen. The residue stream is cooled to form a cooled residue stream. The pressure of the cooled residue stream is reduced to provide a nitrogen-enriched vapor and a methane-rich liquid. The nitrogen-enriched vapor is condensed. The condensed nitrogen-enriched vapor and the methane-rich liquid are fed to a distillation column. The gaseous nitrogen withdrawn from a top of the distillation column is warmed to provide a gaseous nitrogen product stream. The liquid methane withdrawn from a bottom of the distillation column is pressurized. The pressurized liquid methane is vaporized to provide a stream of vaporized methane. The stream of vaporized methane is warmed. The permeate stream and the stream of warmed vaporized methane are combined to provide a gaseous methane product stream.

The method may include one or more of the following aspects.

-   -   said step of cooling the gas mixture, said step of warming         gaseous nitrogen and said step of warming the vaporized liquid         methane are performed at a first heat exchanger.     -   the gaseous nitrogen withdrawn from the top of the distillation         column is further warmed at a second heat exchanger disposed in         fluid communication between the distillation column and the         first heat exchanger and said step of vaporizing liquid methane         is performed at the second heat exchanger.     -   the method further comprises the step of warming the liquid         methane withdrawn from a bottom of the distillation column at a         third heat exchanger before vaporization thereof, wherein:         -   the gaseous nitrogen withdrawn from the top of the             distillation column is further warmed at the third heat             exchanger before being warmed at the second heat exchanger;             and         -   the condensed nitrogen-enriched vapor and the methane-rich             liquid are cooled at the third heat exchanger before being             fed to the distillation column.     -   said step of condensing the nitrogen-enriched vapor is conducted         in a condenser-reboiler operatively associated with the         distillation column.     -   the gas mixture is natural gas obtained from a subterranean         formation.     -   the natural gas comprises from about 60 to about 90 mol %         methane, up to about 25 mol % nitrogen, and from about 0 to         about 10 mol % carbon dioxide.     -   amounts of CO₂ and H₂S are removed from the natural gas prior to         feeding it to the gas separation membrane.     -   the liquid methane from the distillation column is pressurized         with a pump.     -   the gas separation membrane is maintained at a temperature lower         than −20° C.     -   the gas separation membrane is maintained at a temperature of         −50 to −90° C.     -   the gas separation membrane is made of a material selected from         the group consisting of poly(propylene oxide allyl glycidyl         ether) and silicone rubber [poly(dimethyl siloxane).     -   the gas separation membrane is made of a material that has a         methane to nitrogen selectivity of at least 5.     -   the gaseous methane product stream contains less than 6 mol % N₂         and greater than 94 mol % methane.     -   the method further comprises the step of expanding the gaseous         nitrogen product stream and compressing the methane product         stream with a turbo expander.

There is also provided a system for purifying a gas mixture having a majority of methane and a minority of nitrogen, comprising, a source of a gas mixture; a first heat exchanger; a gas separation membrane; a distillation column; and a second heat exchanger. The source of a gas mixture comprises a majority of methane and a minority of nitrogen. The first heat exchanger is adapted to cool a stream of said gas mixture. The gas separation membrane has a feed inlet, a permeate gas outlet, and a residue gas outlet, said feed inlet being in fluid communication with said source via said first heat exchanger. The distillation column has a top and a bottom, a plurality of inlets, a gaseous nitrogen outlet disposed at said column top, and a liquid methane outlet disposed at said column bottom, said plurality of column inlets being in fluid communication with said residue gas outlet. The second heat exchanger is adapted to cool a stream of residue gas from said residue gas outlet, warm a stream of gaseous nitrogen withdrawn from said column top, and vaporize a stream of liquid methane withdrawn from said column bottom. Said first heat exchanger is further adapted to: further warm the stream of gaseous nitrogen warmed at said second heat exchanger; warm a stream of gaseous methane produced by vaporization at said second heat exchanger; and warm a stream of permeate gas from said permeate gas outlet.

The system may include one or more of the following aspects:

-   -   the system further comprises         -   a Joule-Thomson valve in fluid communication between said             residue gas outlet and said plurality of column inlets; and         -   a phase separator comprising an inlet in fluid communication             with said Joule-Thomson valve, a vapor outlet, and a liquid             outlet, said vapor and liquid outlets being in fluid             communication with said plurality of column inlets, said             phase separator being adapted to separate a stream of             residue gas expanded at said valve into a stream of             nitrogen-enriched vapor and a stream of methane-rich liquid.     -   the system further comprises:         -   a Joule-Thomson valve in fluid communication between said             residue gas outlet and said plurality of column inlets;         -   a phase separator comprising an inlet in fluid communication             with said Joule-Thomson valve, a vapor outlet, and a liquid             outlet, said vapor and liquid outlets being in fluid             communication with said plurality of column inlets, said             phase separator being adapted to separate a stream of             residue gas expanded at said valve into a stream of             nitrogen-enriched vapor and a stream of methane-rich liquid             and         -   a condenser-reboiler adapted to condense the stream of             nitrogen-enriched vapor from said phase separator vapor             outlet and vaporize a stream of liquid methane from said             column bottom.     -   the system further comprises a third heat exchanger adapted to         warm the stream of gaseous nitrogen withdrawn from said column         top before warming at said second heat exchanger and warm the         stream of liquid methane withdrawn from said column bottom         before warming at said second heat exchanger.     -   the system further comprises a gaseous methane product conduit         receiving a stream of the permeate gas warmed at said first heat         exchanger and a stream of gaseous methane warmed at said first         exchanger to provide a stream of gaseous methane product.     -   the system further comprises         -   a gaseous methane product conduit receiving a stream of the             permeate gas warmed at said first heat exchanger and a             stream of gaseous methane warmed at said first exchanger to             provide a stream of gaseous methane product; and         -   a turbo expander adapted to expand the stream of gaseous             nitrogen warmed at said first heat exchanger and compress             the stream of gaseous methane product.     -   said gas mixture is natural gas and said source is disposed         within a subterranean formation.     -   the system further comprises a purification unit in fluid         communication between said source and said gas separation         membrane, said purification unit being adapted to remove at         least a portion of CO₂ and H₂S from a stream of said natural gas         from said source by adsorption and/or membrane purification         techniques.     -   the system further comprises a pump adapted to pump a stream of         liquid methane from said column bottom.     -   said gas separation membrane is made of a material selected from         the group consisting of poly(propylene oxide allyl glycidyl         ether) and silicone rubber [poly(dimethyl siloxane).     -   said gas separation membrane is made of a material that has a         methane to nitrogen selectivity of at least 5.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

FIG. 1 is a schematic of a prior art double column system for nitrogen removal from methane.

FIG. 2 is a schematic of a prior art single column system for nitrogen removal from methane.

FIG. 3 is a schematic of the hybrid membrane/cryogenic distillation system according to the invention for nitrogen removal from methane.

DESCRIPTION OF PREFERRED EMBODIMENTS

As best illustrated in FIG. 3, the method and system according to the invention starts with a feed gas 201 containing a majority amount of methane and a minority amount of N₂. The feed gas 201 may be natural gas obtained from a subterranean formation or a methane-containing landfill gas from a landfill. In either case, such a methane-based feed gas 201 typically comprises from about 60 to about 90 mol % methane, from about 0 to about 25 mol % nitrogen, from about 0 to about 10 mol % carbon dioxide (up to about 50 mol % carbon dioxide in the case of a methane-containing gas derived from a landfill), and moisture and other minor substituents. If the feed gas 201 contains undesirable amounts of impurities such as CO₂ and H₂S, it may be pre-treated using a conventional purification unit to remove those impurities and moisture prior to sending it to membrane separation unit 208 (before or after heat exchanger 205). The purification unit may employ any number of well-known adsorption and/or membrane-based purification techniques. A knock-out drum may be utilized to remove heavier hydrocarbons from the gas mixture. Feed gas 201, typically at ambient temperature and a pressure in the range of from about 35 to 80 bar, is cooled to a temperature of less than −20° C. preferably to a temperature of about −50 to about −90° C. in heat exchanger 205.

The cooled feed gas stream 206 is directed to membrane separation unit 208 that includes one or more membrane selectively permeable to methane over N₂. Methane, being the fast gas, permeates through the one or more membranes and the result permeate stream 210 is directed back across heat exchanger 205 thereby warming it to yield warmed permeate stream 298. Depending upon the N₂ content in stream 206, a significant portion of the methane may be separated out in the permeate. For example, at 15% N₂ content, as much as 65% of the feed is permeated through the membrane. The operating temperature of the gas separation unit is maintained at or below −20° C. Preferably, it is maintained at a temperature of about −60 to about −90° C. Typically, the permeate stream 210 contains from about 90 to about 95 mol % methane. A back pressure control valve on the permeate side of the membrane separation unit 208 may be used to control the pressure of the permeate stream 210 (which should be slightly higher than the product pressure). This valve is throttled to adjust the permeate flux and its composition.

In the cooled feed gas stream 206, the N₂, being the slow gas, tends to not permeate through the one or more membranes and thus accumulates in the residue stream 211. Residue stream 211 is cooled to a temperature of about −110° C. at heat exchanger 214. The cooled residue stream 209 is then flashed at valve 220 and directed to phase separator 224 where it is separated into a N₂-enriched vapor stream 228 and a methane-enriched liquid stream 242. The vapor stream 228 is condensed at condenser-reboiler 282 and condensed vapor 249 is optionally cooled at optional heat exchanger 261 and directed in stream 275 as reflux to distillation column 281. The liquid stream 242 is optionally subcooled at optional heat exchanger 261 and also directed in stream 272 to column 281.

Column 281 produces a gaseous N₂-rich stream 269 and a liquid methane-rich stream 278. Typically, stream 269 includes about 5 mol % methane. Typically, stream 278 includes at least about 95 mol % methane and preferably more than 97 mol % methane. Stream 269 is warmed at heat exchangers 261 (optionally), 214, 205 to yield gaseous N₂ product stream 296, typically at a pressure of about 3 to 5 bar.

Liquid methane-rich stream 276 is directed to condenser-reboiler 282 utilizing heat from stream 228 to provide a stream of vaporized or partially vaporized methane 284 to column 281. A liquid methane-rich stream 290 is sent to liquid pump 293.

The stream of liquid methane pumped by pump 293 is optionally directed via stream 265 to optional heat exchanger 261 where it is warmed, but is in any case directed via stream 215 to heat exchanger 214 where it is vaporized and then directed via stream 212 where it is warmed to provide gaseous methane stream 294. Gaseous methane stream 294 is combined with warmed permeate stream 296 at a methane product conduit to provide methane product stream 295. Typically, stream 295 contains less than 6 mol % N₂ and greater than 94 mol % methane. If desired, a turbo expander may be utiized to transfer power from expansion of the N₂ product stream 296 to compression of the methane product stream 295. Whether or not the turbo expander is utilized, the methane product stream 295 typically has a pressure of about 36 bar with a feed gas 201 pressure of about 77 bar.

The patent and non-patent literature in the field of gas separation is replete with details on how to construct or where to procure the membrane separation unit 208, so their details need not be duplicated herein. The membrane or membranes in membrane separation unit 208 may be configured in any way known in the field of gas separation, including a sheet, tube, hollow fiber, etc. Preferably, the membrane is a spiral flat sheet membrane or hollow fiber membrane. Generally speaking, the requisite methane/nitrogen membrane selectivity will depend upon the N₂ content of the cooled feed gas stream 206. At a temperature of −67° C., a selectivity of 7 was sufficient for feed gas stream 206 contents of 15-25% N₂. The selectivity may be modified by changing the temperature of the cooled feed gas stream 206. If a higher selectivity is desired, the temperature should be lowered. The membrane is made of a polymeric material such that, when operated at a temperature of no greater than −20° C., the membrane has a selectivity to methane over N₂ of at least 5, preferably of at least 7. Because the feed gas stream 201 is cooled via heat exchange with streams 294, 296, and 298 at heat exchanger 205, when it enters the gas separation unit 208 via stream 206, it is already at a temperature where the desired selectivity is realized. In other words, greater selectivity is achieved than that realized at relatively warmer temperatures. Suitable polymeric materials include Parel [poly(propylene oxide allyl glycidyl ether)] and silicone rubber [poly(dimethyl siloxane)]. Preferably, it is silicone rubber.

The configurations of the heat exchangers 205, 214, 261 may be any of the known configurations in the field of gas separation, including the shell and tube-type or the brazed type. The brazed exchanger in particular is the preferred choice for this type of process since it can provide an economical configuration for multi streams exchangers.

The patent and non-patent literature in the field of gas separation is replete with details on how to construct or where to procure the Joule-Thomson valve 220, phase separator 224, column 281, condenser-reboiler 282, and pump 293, and as such, they need not be duplicated herein.

Practice of the process yields several advantages.

The hybrid scheme of the invention can treat varying N₂ contents in the feed gas stream with relatively high methane recovery (>97%). The membrane acts as a regulator to optimize the nitrogen content of the feed for distillation by performing a partial separation of the N₂ and methane upstream of distillation. This represents a significant advantage over either a cryogenic-only solution which operates efficiently over a narrow range of feed nitrogen or a membrane-only solution which might not achieve the separation with acceptable recovery.

The hybrid scheme of the invention also lowers capital costs of a system separating N₂ from methane. In comparison to the single or double column systems of FIGS. 2 and 1, the size of the distillation column 281 may be reduced because the membrane reduces the feed sent to column 281. Also, the gas mixture is separated into methane and nitrogen utilizing only a single distillation column.

The hybrid scheme of the invention also results in lower operating costs because the energy requirements, in comparison to the conventional systems, are relatively low. Expansion of compressed gas provides cryogenic temperatures for the distillation column. Thus, external energy for cooling is unnecessary. Cross-exchange of heat of the N₂ product and methane product components with the membrane feed provides the desirable low operating temperature in the membrane, again removing the need for external energy for cooling. Indeed, this process can achieve the separation with no external energy other than the small amount needed for pumping the liquid methane. On the other hand, those skilled in the art will recognize that operating costs are relatively greater for systems utilizing a compressor for compression of gaseous methane, because under most conditions compressing a gas is much more energy-intensive than pumping a liquid.

Preferred processes and apparatus for practicing the present invention have been described. It will be understood and readily apparent to the skilled artisan that many changes and modifications may be made to the above-described embodiments without departing from the spirit and the scope of the present invention. The foregoing is illustrative only and that other embodiments of the integrated processes and apparatus may be employed without departing from the true scope of the invention defined in the following claims. 

1. A method of purifying a gas mixture having a majority of methane and a minority of nitrogen, comprising the steps of: cooling the gas mixture; feeding the cooled gas mixture to a gas separation membrane to provide a permeate stream further enriched in methane and a residue stream further enriched in nitrogen; cooling the residue stream to form a cooled residue stream; reducing the pressure of the cooled residue stream to provide a nitrogen-enriched vapor and a methane-rich liquid; condensing the nitrogen-enriched vapor; feeding the condensed nitrogen-enriched vapor and the methane-rich liquid to a distillation column; warming gaseous nitrogen withdrawn from a top of the distillation column to provide a gaseous nitrogen product stream; pressurizing liquid methane withdrawn from a bottom of the distillation column; vaporizing the pressurized liquid methane to provide a stream of vaporized methane; warming the stream of vaporized methane; combining the permeate stream and the stream of warmed vaporized methane to provide a gaseous methane product stream.
 2. The method of claim 1, wherein said step of cooling the gas mixture, said step of warming gaseous nitrogen and said step of warming the vaporized liquid methane are performed at a first heat exchanger.
 3. The method of claim 2, wherein the gaseous nitrogen withdrawn from the top of the distillation column is further warmed at a second heat exchanger disposed in fluid communication between the distillation column and the first heat exchanger and said step of vaporizing liquid methane is performed at the second heat exchanger.
 4. The method of claim 3, further comprising the step of warming the liquid methane withdrawn from a bottom of the distillation column at a third heat exchanger before vaporization thereof, wherein: the gaseous nitrogen withdrawn from the top of the distillation column is further warmed at the third heat exchanger before being warmed at the second heat exchanger; and the condensed nitrogen-enriched vapor and the methane-rich liquid are cooled at the third heat exchanger before being fed to the distillation column.
 5. The method of claim 1, wherein said step of condensing the nitrogen-enriched vapor is conducted in a condenser-reboiler operatively associated with the distillation column.
 6. The method of claim 1, wherein the gas mixture is natural gas obtained from a subterranean formation.
 7. The method of claim 6, wherein the natural gas comprises from about 60 to about 90 mol % methane, up to about 25 mol % nitrogen, and from about 0 to about 10 mol % carbon dioxide.
 8. The method of claim 7, wherein amounts of CO₂ and H₂S are removed from the natural gas prior to feeding it to the gas separation membrane.
 9. The method of claim 1, wherein the liquid methane from the distillation column is pressurized with a pump.
 10. The method of claim 1, wherein the gas separation membrane is maintained at a temperature lower than −20° C.
 11. The method of claim 1, wherein the gas separation membrane is maintained at a temperature of −50 to −90° C.
 12. The method of claim 1, wherein the gas separation membrane is made of a material selected from the group consisting of polypropylene oxide allyl glycidyl ether) and silicone rubber [poly(dimethyl siloxane).
 13. The method of claim 1, wherein the gas separation membrane is made of a material that has a methane to nitrogen selectivity of at least
 5. 14. The method of claim 1, wherein the gaseous methane product stream contains less than 6 mol % N₂ and greater than 94 mol % methane.
 15. The method of claim 1, further comprising the step of expanding the gaseous nitrogen product stream and compressing the methane product stream with a turbo expander.
 16. The method of claim 1, wherein the gas mixture is landfill gas from a landfill.
 17. A system for purifying a gas mixture having a majority of methane and a minority of nitrogen, comprising: a source of a gas mixture comprising a majority of methane and a minority of nitrogen; a first heat exchanger adapted to cool a stream of said gas mixture; a gas separation membrane having a feed inlet, a permeate gas outlet, and a residue gas outlet, said feed inlet being in fluid communication with said source via said first heat exchanger; a distillation column having a top and a bottom, a plurality of inlets, a gaseous nitrogen outlet disposed at said column top, and a liquid methane outlet disposed at said column bottom, said plurality of column inlets being in fluid communication with said residue gas outlet; and a second heat exchanger adapted to cool a stream of residue gas from said residue gas outlet, warm a stream of gaseous nitrogen withdrawn from said column top, and vaporize a stream of liquid methane withdrawn from said column bottom, wherein said first heat exchanger is further adapted to: further warm the stream of gaseous nitrogen warmed at said second heat exchanger; warm a stream of gaseous methane produced by vaporization at said second heat exchanger; and warm a stream of permeate gas from said permeate gas outlet.
 18. The system of claim 17, further comprising: a Joule-Thomson valve in fluid communication between said residue gas outlet and said plurality of column inlets; and a phase separator comprising an inlet in fluid communication with said Joule-Thomson valve, a vapor outlet, and a liquid outlet, said vapor and liquid outlets being in fluid communication with said plurality of column inlets, said phase separator being adapted to separate a stream of residue gas expanded at said valve into a stream of nitrogen-enriched vapor and a stream of methane-rich liquid.
 19. The system of claim 18, further comprising: a condenser-reboiler adapted to condense the stream of nitrogen-enriched vapor from said phase separator vapor outlet and vaporize a stream of liquid methane from said column bottom.
 20. The system of claim 17, further comprising a third heat exchanger adapted to warm the stream of gaseous nitrogen withdrawn from said column top before warming at said second heat exchanger and warm the stream of liquid methane withdrawn from said column bottom before warming at said second heat exchanger.
 21. The system of claim 17, further comprising a gaseous methane product conduit receiving a stream of the permeate gas warmed at said first heat exchanger and a stream of gaseous methane warmed at said first exchanger to provide a stream of gaseous methane product.
 22. The system of claim 21, further comprising a turbo expander adapted to expand the stream of gaseous nitrogen warmed at said first heat exchanger and compress the stream of gaseous methane product.
 23. The system of claim 17, wherein said gas mixture is natural gas and said source is disposed within a subterranean formation.
 24. The system of claim 23, further comprising a purification unit in fluid communication between said source and said gas separation membrane, said purification unit being adapted to remove at least a portion of CO₂ and H₂S from a stream of said natural gas from said source using adsorption and/or membrane purification techniques.
 25. The system of claim 17, further comprising a pump adapted to pump a stream of liquid methane from said column bottom.
 26. The system of claim 17, wherein said gas separation membrane is made of a material selected from the group consisting of poly(propylene oxide allyl glycidyl ether) and silicone rubber [poly(dimethyl siloxane).
 27. The system of claim 17, wherein said gas separation membrane is made of a material that has a methane to nitrogen selectivity of at least
 5. 28. The system of claim 17, wherein said gas mixture is landfill gas and said source is a landfill. 